A series of preliminary studies have suggested that smoking behavior may be associated with certain gonadal deficits (testicular varicoceles; low sperm count; reduced sperm motility; and low levels of pubic hair development; which suggests a low level of testosterone stimulation). In addition, cognitive and electroencephalographic indices known to be sensitive to testosterone administration also differentiate Smokers from Nonsmokers. Some of these differences are found only in individuals who start to smoke at or before age 15 (Early Smokers). This suggests that the onset of smoking may have differential effects related to the stage of development at which it occurs. If these preliminary findings can be confirmed, a new area of risk associated with smoking will have been identified. The proposed research would attempt to confirm the above observations in 300 young adult male volunteers divided into 6 groups in a manner which permits the assessment of the effects of smoking, age of onset of smoking, duration of smoking, and age. The specific variables to be studied are left testicular varicoceles, pubic hair development, seminal fluid measures, cognitive test performances, and EEG "driving" responses to photic stimulation. Since some of the above measures suggest that a difference in testosterone stimulation may exist between Smokers and Nonsmokers, the blood metabolic clearance and production rates of testosterone will be studied in 20 Early Smokers, 20 Late Smokers, and 40 Nonsmokers.